This invention relates to a novel class of optical glasses, and more particularly, to such glasses which exhibit good stability with respect to changes in climatic conditions, low staining susceptibility and can be ground, polished and pressed like ordinary silicate glasses. These glasses have the same optical position as lanthanum crown glasses (LaC) and lanthanum flint glasses (LaF), but exhibit a higher partial dispersion P.sub.g,F than prior glasses.
Normal commercial glasses, in the optical positional range of the LaC and LaF glasses, have a partial dispersion P.sub.g,F = (n.sub.g - n.sub.F)/n.sub.F - n.sub.C) which is considerably reduced, by comparison, with the normal partial dispersion, as calculated from the Abbe equation EQU P.sub.g,F = 0.6438 - 0.001682v.sub.d
and set forth, for example, in the Jenaer Glaswerk Schott & Gen. catalogue for optical glass no. 3050/1966/II 5.
Prior art glasses within the optical positional range of LaC and LaF glasses are characterized by the passage of a straight line through each of the following limiting values (as illustrated in FIG. 1):
______________________________________ n.sub.d = 1.642 v.sub.d = 58.0 to n.sub.d = 1.699 v.sub.d = 46.0 n.sub.d = 1.642 v.sub.d = 58.0 to n.sub.d = 1.652 v.sub.d = 58.5 n.sub.d = 1.652 v.sub.d = 58.5 to n.sub.d = 1.702 v.sub.d = 54.6 n.sub.d = 1.702 v.sub.d = 54.6 to n.sub.d = 1.742 v.sub.d = 44.0 n.sub.d = 1.742 v.sub.d = 44.0 to n.sub.d = 1.699 v.sub.d = 46.0 ______________________________________
Each of these prior art glasses exhibit particularly low P.sub.g,F values.
Glasses having the lowest P.sub.g,F values and most extreme optical positions (e.g. n.sub.d =1.697; v.sub.d =55.4) consist essentially of B.sub.2 O.sub.3, CaO, La.sub.2 O.sub.3, ZrO.sub.2 and less than 10 percent by weight of SiO.sub.2. In addition to these ingredients, these glasses can also contain MgO, SrO, PbO, ZnO, CdO, TiO.sub.2, Al.sub.2 O.sub.3, Nb.sub.2 O.sub.5, and WO.sub.3 for varying the n.sub.d and v.sub.d values and for improving the crystallization stability and chemical resistivity of the glasses.
In prior art glasses, when the SiO.sub.2 content was raised to above 10 percent by weight, with a corresponding reduction in the amount of B.sub.2 O.sub.3 present, the optical positional range became smaller and hence an increase in the crystallization tendency and a lowering of the v.sub.d value with a given n.sub.d value of the glass occurred. One attempt to avoid the tendency of the glass to crystallize was to partially replace CaO and La.sub.2 O.sub.3 with BaO and ThO.sub.2, respectively. However, this attempt was not successful because the v.sub.d was considerably low and the P.sub.g,F values were also considerably below the aforementioned normal partial dispersion value. A need therefore has existed for optical glasses having the same optical position as LaC and LaF glasses but with higher partial dispersion P.sub.g,F than prior known glasses of the same optical position.